Optical amplifiers such as erbium doped fiber amplifiers (EDFAs) or semiconductor optical amplifiers (SOAs) are employed in optical transmission systems to amplify optical signals along the optical transmission path. In one application, optical amplifiers are used in optical receivers prior to photo detection. While an optical signal is being amplified, unwanted Amplified Spontaneous Emission (ASE) noise may be generated. In many cases, the ASE noise will degrade receiver performance.
For example, if an optical amplifier has a wide amplification bandwidth and ASE noise is not sufficiently optically filtered after amplification, ASE noise may present a problem. For those cases, additional filtering of the ASE noise is necessary to achieve good sensitivity of the pre-amplified receiver. The total ASE power is one figure of merit for the sensitivity that can be achieved for this pre-amplified receiver.
Known noise filtering is generally performed using one of gain flattening filters, fixed wavelength band pass filters with the signal centered on the band pass filter, band-edge filters for ASE filtering, or continuously tunable band pass filters. These known noise filters have certain disadvantages.
For example, a gain flattening filter is used to achieve some degree of ASE suppression. But the total ASE power seen at the output is still unacceptably high for large gain/wide bandwidth amplifiers. Fixed-wavelength filters do not work if the signal wavelength changes and the receiver is wavelength independent over a large wavelength range (e.g., WDM systems). Like fixed wavelength filters, a band-edge filter limits the achievable wave length range. In this case, it is limited to some range outside the gain peak and thus the ASE peak of the optical amplifier. Regarding continuously tunable band pass filters, although technically sound, this approach is costly and bulky in deployed systems.
In the laboratory, one would use an optical amplifier and a general filter to remove, or at least minimize, ASE noise. For system implementation, one would use gain flattening filters to remove some, if not all, of the ASE noise between the stages of a multistage optical amplifier. Here, again, one only can attenuate the ASE noise to some degree using these types of filters. One cannot remove all of the ASE noise.
Thus, there is a need for improved ASE noise suppression in broadband optical telecommunication systems.